Polyimides are an important class of polymeric materials and are known for many desirable performance properties. These properties include high glass transition temperatures, good mechanical strength, high Young's modulus, good UV durability, and excellent thermal stability. As a result of their favorable properties, polyimide compositions have become widely used in many industries, including the aerospace industry, the electronics industry and the telecommunications industry.
In the electronics industry, polyimide compositions are used in applications such as forming protective and stress buffer coatings for semiconductors, thermal insulating coatings, dielectric layers for multilayer integrated circuits and multi-chip modules, high temperature solder masks, bonding layers for multilayer circuits, final passivating coatings on electronic devices, and many others. In addition, polyimide compositions may form dielectric films in electrical and electronic devices such as motors, capacitors, semiconductors, printed circuit boards and other packaging structures. Polyimide compositions may also serve as an interlayer dielectric in both semiconductors and thin film multichip modules. The low dielectric constant, low stress, high modulus, and inherent ductility of polyimide compositions make them well suited for these multiple layer applications. Other uses for polyimide compositions include alignment and/or dielectric layers for displays, and as a structural layer in micromachining applications. Electronic components using polyimide films are used in many other industries.
Polyimides have many different uses in the aerospeace industry, the automotive industry, the rail industry, the natural gas industry, and others. Polyimides can be used as high temperature adhesives, thermal insulations, protective coatings or layers, membranes, gaskets, and a wide variety of other uses.
The increased complexity of the applications for polyimides has created a demand to tailor the properties of such polyimides for specific applications. Compounds or moieties incorporated into a polyimide or other polymer can change the properties of that polymer. For example, dyes can be added to a polymer to change the color, and ultra violet (UV) stabilizers can be added to increase resistance to damage from UV light. Many other compounds can be added to a polymer to change various properties.
Many different compounds can be added to polymers to change the polymer properties, and these compounds can be added in different ways. The added compounds can be covalently bonded to the polymer, dissolved or suspended in the polymer, or otherwise included in the polymer (such as with ionic bonding.) Often, an added compound will change more than one property, so controlling one property independently from a second property can be challenging. Some polymer uses require specific ranges for several different properties, and controlling the measured value of one property can compete with controlling the value of a different property.
Aerogels are well-known in the art for their low-density and effectiveness as thermal insulators. As used herein, “aerogel” is defined as a material produced from a gel wherein the liquid component of the gel has been replaced with a gas, “micropores” is defined as pores with diameters less than 2 nm, and “mesopores” is defined as pores with diameters between 2 nm and 50 nm. Aerogels consist of a highly porous network of micropores and mesopores. The pores of an aerogel can frequently account for over 90% of the volume when the density of the aerogel about 0.05 gram/cc. Aerogels are usually prepared from silica-based materials, as well as from polymers. For both types of materials, aerogels are generally prepared by a supercritical drying technique to remove the solvent from a gel (a solid network that encapsulates its solvent) such that no solvent evaporation can occur, and consequently no contraction of the gel can be brought by capillary forces at its surface. For polymer-based aerogels, therefore, aerogel preparation typically proceeds as follows: (1) polymerization of the polymer gel; (2) formation of the gel; and (3) solvent removal by supercritical drying. Supercritical carbon dioxide drying is a sensitive, time consuming procedure that requires expensive machinery to accomplish and requires the handling of corrosive supercritical solvents which are damaging to the environment. Accordingly, there exists a need in the art for a quick, easy, robust method of producing an aerogel that eliminates the need for supercritical carbon dioxide drying.